Lyme disease is the most prevalent arthropod-borne disease of humans in the United States and many other countries throughout Europe and Asia. The objectives of this project are to (1) use recombinant DNA techniques to express specific antigens of Borrelia burgdorferi and B. hermsii to improve the serodiagnosis of Lyme disease and relapsing fever, (2) characterize at the molecular level, isolated of the Lyme disease and relapsing fever spirochetes from a wide range of biological and geographical sources, (3) examine adaptive molecular responses produced by B. burgdorferi and B. hermsii during infection in ticks. During the last year, we directed most of our efforts towards the first and third objectives. Previously we demonstrated the differential expression of outer surface proteins (Osps) by spirochetes in unfed and fed ticks. Spirochetes in the unfed nymphal ticks~ midgut expressed OspA but not OspC. In contrast, spirochetes in ticks having completed feeding on a mammal synthesized OspC. This change on the spirochete~s surface in the feeding tick, controlled in part by temperature and blood, has relevance to both vaccine development and serodiagnosis of Lyme disease. We have examined the differential expression of OspA and OspC in more detail during tick feeding and have also examined the synthesis of these proteins during the transmission of spirochetes from infected mammals to previously uninfected ticks. When infected nymphs feed, the spirochetes rapidly change during the first 48 hrs of tick attachment. Just prior to attachment, no spirochetes are expressing OspC, while 2 days later, approximately 50% of the bacteria now have OspC on their surface. Corresponding to this is a drop in the proportion of spirochetes expressing OspA, from 100% just prior to to the mammalian host. We have also examined spirochetes for expression of OspA and OspC during their transfer from infected mice to previously uninfected larvae and nymphs. Bot feeding to about 50% after 4 days of tick feeding. After feeding and tick dropoff, the spirochetes rapidly convert back to the prefeeding state, with 100% of the bacteria expressing OspA and none expressing OspC. These data support further the hypothesis that OspC is involved with the spirochetes dissemination and transmission from the tick~s midgut h larvae and nymphs rapidly acquire spirochetes, even during the first 24 hr before any detectable blood has been ingested. In all ticks examined at 24, 48, and 72 hr of attachment, all spirochetes express OspA and none expresses OspC. The lack of expression of OspC in these feeding ticks is in striking contrast to what we observed previously for spirochetes in ticks already infected prior to feeding. Future studies will attempt to identify the factors controlling the regulation of OspA and OspC.